全新java初学者实践教程10(java SE5.0版)
A\8}|r(>9E jdk5的集合类
,^#yo6- y ]?V~% yR(x+Gs{] RV@*c4KvO+ 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
>4t+:Ut: 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
R<OI1,..r [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
AU2i%Q! giz7{Ai [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
c("_bOAT BcI|:qv| [font="Times](1) Set
dAy?EO0\7 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
q$=#A7H>3) rRFhGQq1m ([font="Times]2)[font="Times]List
%2v4<icvq 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
Ja9e^`i; _%QhOY5tv" =:~R=/ZXk [font="Times] ([font="Times]3)[font="Times]Map
K4kMM*D [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
:0h_K eUg~)m5G =ZqT3_ [font="Times] ([font="Times]4)[font="Times]Queue
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A [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
x&0vKo; [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
fk&8]tK4 z*-2.}&U< irfp!(r 实践:[font="Times] Set举例
)rC6*eR [font="Times] import java.util.*;
AJ=qn a [font="Times]public class SetExample {
j:VbrR [font="Times] public static void main(String[] args) {
t2)rUWg [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
"men [font="Times] set.add("one");
OynXkH]0T+ [font="Times] set.add("second");
4T ~} [font="Times] set.add("3rd");
i*Z"Me [font="Times] set.add(new Integer(4));
"yS _s [font="Times] set.add(new Float( 5.0F ));
s=;uc]9g [font="Times] set.add("second");
3U0`,c\ao* [font="Times] set.add(new Integer(4));
}|pwz [font="Times] System.out.println(set);
,LnII [font="Times] }}
P(f0R8BE Rd#WMo2Xd [font="Times]List举例:
N(Tz%o4 [font="Times] import java.util.*;
%4gg@Z9 [font="Times]public class ListExample {
~GZY 5HF [font="Times] public static void main(String[] args) {
W>j !Q^? [font="Times] List list = new ArrayList();
V)[@98T_4? [font="Times] list.add("one");
uo@n(>}EL [font="Times] list.add("second");
GU( _ [font="Times] list.add("3rd");
NeE
t [font="Times] list.add(new Integer(4));
:qd`zG3 [font="Times] list.add(new Float( 5.0F ));
*Qg _F6y [font="Times] list.add("second");
gMzcTmbc8 [font="Times] list.add(new Integer(4));
zN^n]N_? [font="Times] System.out.println(list);
mjW8Q\D [font="Times] }}
f,018]| sTn<#l6 -;^j:L{ [font="Times]Map举例
ww],y@da [font="Times]import java.util.Map;
ur:3W6ZKl [font="Times]import java.util.HashMap;
n[lJLm^(_C [font="Times]import java.util.Iterator;
w5Y04J [font="Times]import java.io.FileReader;
YQj 2 [font="Times]
^hZ0IM [font="Times]public class MapExample {
~a ]+#D [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
6ZP(E^. [font="Times] Map word_count_map = new HashMap();
Nn!+,;ut [font="Times] FileReader reader = new FileReader(args[0]);
t/VD31 [font="Times] Iterator words = new WordStreamIterator(reader);
xMTKf+7 [font="Times]
wl#@lOv-P [font="Times] while ( words.hasNext() ) {
&,{fw@#)_ [font="Times] String word = (String) words.next();
au:
fw [font="Times] String word_lowercase = word.toLowerCase();
X= 5xh [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
*U&0<{|T [font="Times]if ( frequency == null ) {
0i!uUF [font="Times] frequency = new Integer(1);
dfKF%27 [font="Times] } else {
RtTJ5@V( [font="Times] int value = frequency.intValue();
Oo5w?+t [font="Times] frequency = new Integer(value + 1);}
2bw_IT [font="Times] word_count_map.put(word_lowercase, frequency);
TaKLzd2 [font="Times] }
49GkPy#]L= [font="Times] System.out.println(word_count_map);
D$
dfNiCH [font="Times] }}
zzTfYf) hI]Hp3S [>8}J" [font="Times]Queue举例:
RJ 8+h [font="Times]import java.io.IOException;
<D<4BnZ( [font="Times]import java.io.PrintStream;
,(d)Qg [font="Times]import java.util.LinkedList;
G_bG [font="Times]import java.util.Queue;
SUCMb8 [font="Times]
>,gg5<F-E [font="Times]public class QueueTester {
/uPcXq:L~ [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
l{I6&^!KS [font="Times] public QueueTester() {q = new LinkedList();}
!h"Kq>9T [font="Times]public void testFIFO(PrintStream out) throws IOException {
DFZkh^PFd [font="Times] q.add("First");
DS+}UO [font="Times] q.add("Second");
]
o*#t [font="Times] q.add("Third");
>`Y.+4mE [font="Times] Object o;
KHe=O1 %QO [font="Times] while ((o = q.poll()) != null) {
ehzM)uK [font="Times] out.println(o);}}
-lbm*
-( [font="Times] public static void main(String[] args) {
|=Eo?Q_ [font="Times] QueueTester tester = new QueueTester();
s.bc>E0
[font="Times] try { tester.testFIFO(System.out);
Xe6w| [font="Times] } catch (IOException e) {
]zj9A]i:a [font="Times] e.printStackTrace(); } }}
ciBP7>':: R.jIl@p RQ vft 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
^o^H3m 总结:
6G1@smP 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
qkt0**\ 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。